In general, mobile magnetic sensors must discriminate the relatively very small DC vector magnetic anomaly fields that emanate from magnetically polarized targets while the sensor's platform motion in the large magnetic field of Earth can cause large non-target-related changes in vector field measurements. The large, non-target-related changes in measured field components can overwhelm the relatively small target fields (or, “signatures”) and greatly reduce the effective target detection range of vector magnetometers. Consequently, mobile magnetic sensing applications have generally required the use of sensors that either measure the scalar magnitude of the magnetic field (i.e., “scalar total field magnetometers”) or measure the spatial gradients of field (i.e., “tensor gradiometers” that measure the rate of change of the magnetic field with distance). While scalar magnetometers provide relatively long target detection range, they have relatively poor target localization or homing capabilities. On the other hand, tensor gradiometers can provide very good target localization capabilities at the expense of target detection range.
Since scalar total field magnetometers are minimally affected by changes in sensing platform orientation, they have a long history of use in mobile magnetic sensing applications such as aerial geological surveys and antisubmarine warfare. Aerial applications typically involve the use of single scalar magnetometers that are either towed below the aircraft or mounted in a “stinger” located behind the tail of the aircraft. In addition, single sensors or arrays of two or three scalar magnetometers are commonly towed behind water vessels to search for smaller targets such as underwater and buried mines. For aerial and maritime applications, the scalar total field magnetometers generally require multiple passes over a target region (with the targets usually located below the sensor platform) in order to approximately “map out” the location of a target. For example a commercially available product by Marine Magnetics, Inc., Ontario, Canada, uses an array of three scalar magnetometers in a multi-pass “raster scan” search modality to localize and classify magnetic targets. However, multi-pass target-mapping modalities are not efficient for autonomous sensing platforms equipped with limited power capabilities. That is, a small autonomous vehicle's magnetic sensor system must be able to directly guide the vehicle so that it can directly home in on a target. However, the following specific characteristics of scalar total field sensors and the total magnetic field have heretofore impeded the use of total field sensors for directly homing in on magnetic targets:
(i) The magnitude of total field that surrounds a magnetic anomaly is a complex function of sensor-target distance and target orientation relative to the Earth's magnetic field. In particular, in some regions around a given target, the total field may decrease as a sensor moves closer to the target while in other regions around the same target the total field may increase as sensor-target distance decreases. Therefore, total field data heretofore have not provided a robust basis for homing in on magnetic targets. Here, the term “robust” is defined to mean a magnetic quantity or parameter whose value always behaves in a well defined way, namely, a robust quantity always increases as sensor-target distance decreases and decreases as sensor-target distance increases.
(ii) Scalar magnetometers essentially only measure the component of magnetic anomaly field that is parallel to the Earth's background field. Since they do not measure the full complement of target localization information that is implicit in the vector magnetic anomaly field, they normally require multiple passes of the sensor over a target region in order to map out an approximate localization of a target.
In summary, prior art approaches to magnetic anomaly guidance either have used relatively long-range, total-field sensor arrangements that lack direct target homing capability, or they have used magnetic gradiometers with good guidance capabilities but with a relatively short target detection range in comparison to the total field approach.